This disclosure relates generally to diffractive optical elements, and more particularly, but not exclusively, to a method, apparatus, and system for measuring optical phase and amplitude properties of a diffractive optical element that may be used in the production of Bragg gratings in optical waveguides.
Diffraction gratings and other diffractive optical devices have been developed for many applications. For example, diffraction gratings have been suggested for use in data routing in conjunction with optical communication systems. Fiber Bragg gratings (xe2x80x9cFBGsxe2x80x9d) have been developed for applications including wavelength selection and routing in optical communications, as well as other applications in optical sensors and in optical remote sensing.
Current methods of producing diffraction gratings may be based on holographic techniques, and FBGs may be made by exposing an optical fiber to an interference pattern produced with optical radiation at wavelengths that produce changes in the refractive index of a fiber. In one method, a mask, configured to provide a selected interference pattern, may be provided. Ultraviolet radiation at wavelengths that are typically between about 150 nm and about 400 nm may then be directed toward the mask and a fiber, in which a FBG is to be formed, may be placed in the interference pattern. The fiber may be exposed to the interference pattern for a time period sufficient to produce index of refraction changes of a selected magnitude and in a spatial pattern corresponding to the interference pattern.
While current methods for producing FBGs using masks can be simple to implement and have adequate manufacturing throughput, the properties of the resulting FBGs depend on the properties of mask used to produce the interference pattern. Such masks and other diffractive structures may be characterized with two beam interferometric methods in which an optical field produced by light transmitted through the mask is interfered with a reference plane wave. The resulting interference pattern may then be analyzed to provide phase information about the phase of the transmitted optical field. However, such methods have significant disadvantages. It is generally desirable to measure the transmitted phase front in the near field at distances from the diffractive structure that range from a few micrometers to a few millimeters. Configuring two beam interferometers for measurements at such near field distances is difficult. In addition, two beam interferometric methods generally require ultra-stable environments to eliminate phase noise due to mechanical vibrations or variations in refractive indices experienced by either an optical signal field (i.e., the optical field produced by the diffractive structure under test) or the reference optical field.